1. Field of the Invention
This invention relates to battery separator material and more particularly to battery separators which fully envelope the electrolytic plates.
2. Description of the Prior Art
Electrolytic cells (i.e. batteries) formed by spaced apart metal plates connected in series for storage of electric energy are useful for a variety of purposes. The plates of the electrolytic cells tend to form an active substance on the surface of the electrodes. When the active substance drops off, battery capacity drops, and also the degradation of the metal electrodes forms products including metal salts and other conductive substances which tend to bridge between two electrodes, thereby short circuiting the cell. These salts and degraded metal products are primarily due to the highly acidic environment in storage batteries.
In order to protect the electrode plates, separators have been used between the plates which are porous, thereby allowing ionic exchange through the separator while providing adequate separation between the cells to prevent short circuiting. Typically, these separators are constructed of cellulose fiber or fused polyolefin sheets. Exemplary of battery separators are those disclosed in U.S. Pat. Nos. 2,973,398; 3,890,184; 4,024,323; 4,055,711; 4,113,927; 3,753,784; 3,694,265; 3,773,590; 3,351,495; 3,340,100; 3,055,966; 3,205,098 and 2,978,529.
Battery separators having improved properties have been required with the introduction of what is known as the "maintenance free battery." The maintenance free battery is one which is a sealed unit and does not require the intermittent addition of water thereto. Typically, the battery separator in a maintenance free battery is preferably an envelope sealed on at least three sides in order to prevent bridging with metal salts between electrodes. A primary requirement of these battery separators is that they must have sufficient porosity in order for the ionic exchange to occur while the pores of the separator must be sufficiently small to prevent the migration of heavy metal ions and, consequently, bridging which short circuit the cell.
Further requirements of a battery separator for a lead-acid battery are resistance to acid and oxidative and reductive chemical reactions because of the highly acidic environment within the battery. Also, separators should have as low an electrical resistance as possible to provide good cold discharge performance.
In accordance with the invention, a battery separator material is provided which has excellent filtering, electrical, chemical and physical properties which is readily fabricated and may be folded and heat sealed along the edges in the form of an envelope to act as a battery separator, particularly in a maintenance free battery.